1. Field of the Invention
This invention relates generally to an echo canceller, and more specifically, it relates to an echo canceller used with an adaptive filter.
2. Description of the Background Art
An echo canceller may be used with, for example, a hands free telephone, a TV conference communication system and so on in order to cancel echo signals produced by sound coupling. The echo canceller applies an adaptive algorithm to cope flexibly with a change in an echo path and external noises. Examples of such an adaptive algorithm are a least-mean-squares (LMS) algorithm and a normalized least-mean-squares (NLMS) algorithm, which are described in xe2x80x9cOutline of Adaptive Filter,xe2x80x9d Journal of the Acoustical Society of Japan, Vol. 48, No. 7, Pages 489-492 (1992). These adaptive algorithms are used in the adaptive filter of the echo canceller because the amount of renewal (i.e., adaptive) calculations are relatively small and the renewal operations are stable.
In general, a longer echo path necessitates more tap coefficients for the adaptive filter. For this reason, the adaptive filter must calculate a large number of tap coefficients and have a large memory for the renewal calculations. Furthermore, a larger initial delay for the echo path necessitates more required calculations for the adaptive filter. This problem occurs in spite of the type of adaptive algorithms used.
To overcome this problem, a techniques has been suggested for reducing the amount of the renewal calculations by eliminating some tap coefficients corresponding to the initial delay based on a characteristic of an impulse response of the echo path. To eliminate the tap coefficients corresponding to the initial delay, it is necessary to measure the initial delay of the impulse response. However, for mobile radio communication system, it is difficult to measure correctly the initial delay because the initial delay is always changing.
Therefore, it is an object of the present invention to provide an echo canceller for reducing the amount of the renewal calculation by eliminating some tap coefficients corresponding to the initial delay based on a characteristic of an impulse response on the echo path. That is, to measure the initial delay, an echo canceller having an adaptive algorithm for canceling an echo signal comprises an adaptive filter, a renewal controller and an initial operation controller. The adaptive filter generates a pseudo-echo signal by renewing tap coefficients, based on a far-end input signal, a near-end input signal and a residual signal. The renewal controller determines a time average of a renewal amount for each tap coefficient and estimates an initial delay of the echo signal by detecting the time average indicating a maximum renewal amount. The initial operation controller operates the adaptive filter to perform an initial operation to eliminate the tap coefficients corresponding to the initial delay and to renew the remaining tap coefficients corresponding to the echo signal, when power of the far-end input signal higher than a predetermined first threshold level and power of the near-end input signal is lower than a predetermined second threshold level. Accordingly, the adaptive filter generates the pseudo-echo signal based on the remaining tap coefficients.